1. Field of the Invention
The present invention relates to a liquid electrophotographic color printer using a developer composed of a powder toner and a liquid carrier, i.e. a solvent, and more particularly, to a squeezing apparatus of a liquid electrophotographic color printer for removing both the carrier of the developer and the residual developer left after a developing operation.
2. Description of the Prior Art
Generally, a liquid electrophotographic color printer includes a belt type photosensitive medium 1, as shown in FIG. 1. The photosensitive medium 1 is wound around rolls 2, 3, 4 and travels in a certain path in a body of the printer (not shown). Around the photosensitive medium 1, an electric charging unit 10, exposure unit (not shown), developing units 20a, 20b, 20c, 20d, drying unit 30, and transfer/fixing unit 40 are disposed. Further, developer feeding devices (not shown) adjacent to the developing units 20a, 20b, 20c, 20d are disposed to feed the developer of a certain density to the developing units 20a, 20b, 20c, 20d. 
An electrostatic latent image is formed on the photosensitive medium 1 by the exposure unit (not shown), and is developed by the developing units 20a, 20b, 20c, 20d which jet the developer on to the photosensitive medium 1, separate most of the carrier from the developer to retain the toner of the developer at the electrostatic latent image area of the photosensitive medium 1. The carrier is removed during the developing operation since excess carrier at the image formed on the photosensitive medium 1 by the toner, hinders an image transfer of the transfer/fixing unit 40. The residual carrier which is not removed by the developing units 20 is removed while the photosensitive medium 1 passes through the drying unit 30.
Accordingly, as shown in FIG. 2, each one of developing units (e.g., developing unit 20a) includes a developer jetting nozzle 22, developing roll 24, developing backup roll 26, developer recovery tank 28, and a squeezing device 60 for removing the carrier. Here, the squeezing device 60 includes a squeezing roll 62 and a squeezing backup roll 64.
The squeezing device 60 is described below in greater detail with reference to FIG. 3.
As shown in FIG. 3, the squeezing roll 62 is disposed to be selectively forced against a surface of the photosensitive medium 1, while the squeezing backup roll 64 is disposed to be selectively forced against the opposite surface of the photosensitive medium 1. Further, a pair of forcing blocks 66b, 66c are disposed on a shaft 61 of the squeezing roll 62 to force the squeezing roll 62 against the squeezing backup roll 64 with a certain force, and each one of a pair of the airjet nozzles 68b, 68c are disposed at both sides of the squeezing roll 62. The air jet nozzles 68b, 68c jet air toward both ends of the squeezing roll 62, thereby preventing the developer from undesirably attaching to the ends of the squeezing roll 62 and transferring to a next color.
Further, the squeezing device 60 includes a driving section 70 for reverse-rotating the squeezing roll 62 in a drip line removing mode, and a squeezing blade 72 for removing ink on the squeezing roll 62.
The squeezing device 60, constructed as above, squeezes the carrier out of the developer applied on the surface of the photosensitive medium 1 by its passive-rotation while forcing against the surface of the photosensitive medium 1 with a certain force. Here, the squeezing roll 62 is forced by the force approximately of 20 kgf.
After the printing operation finishes, the drip line removing mode starts. During the drip line removing mode, less force is applied to the squeezing roll 62 than during the printing mode. Here, the driving section 70 rotates the squeezing roll 62 in contact with the squeezing blade 72, in the reverse direction, attracting unnecessary ink from the photosensitive medium 1. The unnecessary ink on the reverse-rotating squeezing roll 62 is removed by the squeezing blade 72 forced against the outer circumference of the squeezing roll 62.
Meanwhile, in the squeezing apparatus 60 of a liquid electrophotographic color printer having the above operation, slipping should not occur when the squeezing roll 62 is passive-rotated while being forced against the photosensitive medium 1. If slipping occurs, a smeared image is formed on the image area of the photosensitive medium 1, resulting in a deterioration of print quality. Accordingly, in order to increase a frictional force between the squeezing roll 62 and the photosensitive medium 1, the squeezing roll 62 is usually made of a material having a high friction coefficient.
While the squeezing roll 62, made of a material having a high friction coefficient, has an advantage of preventing slippage, it also has a disadvantage, i.e., a high peel force. A certain amount of toner is attached to the squeezing roll 62 with the high peel force, when the carrier is squeezed out from the developer on the photosensitive medium 1, causing squeezing offset error. Further, with the high friction coefficient of the squeezing roll 62, the image area of the photosensitive medium 1 is considerably damaged during the reverse-rotation of the squeezing roll 62 for removing the drip line.
Accordingly, slipping should be prevented and the proper peel force should be maintained when the squeezing roll 62 is driven by the photosensitive medium 1.
Conventionally, the squeezing roll 62 has a reduced peel force while having a certain degree of friction force from a TEFLON or silicon coating on the surface of the squeezing roll 62. That is, in order to prevent image smearing or squeezing offset error, a material having a certain amount of friction force which can maintain a low peel force, is employed.
In the conventional squeezing apparatus described as above, however, the squeezing is performed while the toner is left on the entire contact area between the photosensitive medium 1 and the squeezing roll 62. Even though the force of 20 kgf is applied to the photosensitive medium 1 and the squeezing roll 62, there are problems caused by the load produced by a roll deformation during a nip formation, load between a supporting structure and a roll shaft caused by the pushing force, and load of the driving elements of the driving section 70 for the drip line removing mode. The problems result in a small, but an unignorable relative speed difference between the photosensitive medium 1 and squeezing roll 62. There is a maximum 150 gm/sec relative speed difference, slightly varying depending on the colors used. Consequently, there are problems such as image smearing.
Further, the structure of the conventional squeezing roll 62, made of a material having a low peel force and a certain amount of friction coefficient, has limitations, and accordingly, a printing environment can increase the peel force of the squeezing roll 62, or the squeezing roll 62 may still slip. As a result, the problems such as the squeezing offset error caused by the increasing peel force of the squeezing roll 62, and the image smearing caused by the slip of the squeezing roll 62 can not be prevented.
The present invention has been made to overcome the above problems of the prior art, and accordingly, it is an object of the present invention to provide a squeezing apparatus of a liquid electrophotographic color printer for removing the carrier from a developer without causing any damage to a developed image on a photosensitive medium by maintaining a proper (e.g., small) amount of peel force and preventing any slips.
Another object of the present invention is to provide a squeezing apparatus of a liquid electrophotographic color printer for minimizing any possible damage to a photosensitive medium due to a squeezing roll during a drip line removing mode by decreasing a friction coefficient of the squeezing roll forced against an image area of the photosensitive medium.
The above objects are accomplished by the squeezing apparatus of a liquid electrophotographic color printer according to the present invention, including a squeezing roll for squeezing out and removing the carrier from the developer which is applied on an image area of the photosensitive medium by its passive-rotation while being forced against the photosensitive medium by a certain force.
The squeezing roll has a dual structure, since it includes a squeezing roll section, made of a material having a low peel force, which comes in contact with the image area of the photosensitive medium, and a pair of friction roll sections made of a material of a high friction coefficient which are coaxially disposed at both sides of the squeezing roll section and come in contact with a non-image area of the photosensitive medium.
The squeezing roll section is made of a material which has a low peel force such as silicon, or others, and the friction roll sections are made of a material with a high dry-friction coefficient such as urethane, or other materials. Accordingly, since the squeezing roll is passive-rotated while being forced against the photosensitive medium by the friction roll section under a high friction force, slipping does not occur. Also, since the squeezing roll section having a low peel force comes into contact with the image area of the photosensitive medium, the carrier can be exclusively squeezed out without causing any damage to the image.
The present invention prevents the squeezing roll from slipping and the toner of the developer from attaching onto the squeezing roll, to prevent image blur and squeezing offset error.
Meanwhile, TEFLON can be coated on the outer surface of the squeezing roll section. The squeezing roll section, however, can be made of any material as long as the material has a low peel force. Likewise, the friction roll section can be made of urethane, or any other material that has a high dry-friction coefficient.
Further, in the squeezing apparatus according to the present invention, each one of the friction rolls are disposed at opposite ends of the squeezing roll section, with a certain gap between each end of the squeezing roll section and the respective friction roll section. The developer is applied on the squeezing roll section, and the gaps between the squeezing roll section and the friction roll sections serve as an oil fence for preventing the developer from attaching on to the friction roll sections. Accordingly, the friction roll sections staying in a dry-friction condition at the non-image areas of the photosensitive medium can prevent slipping more efficiently.
Further, in the squeezing apparatus according to the present invention, each nozzle of a pair of airjet nozzles is disposed in a gap between the squeezing roll section and the respective friction roll section, to prevent the developer from flowing to the ends of the squeezing roll section. Further, a pair of forcing blocks are disposed on the shaft of the squeezing roll. One end of the shaft of the squeezing roll is connected with a driving section for reverse-rotating the squeezing roll in the drip line removing mode. Also, a squeezing blade is disposed to selectively come in contact with the outer circumference of the squeezing roll section for removing ink which is attached onto the squeezing roll section during the drip line removing mode.